Objectives Determine whether female marmosets housed with their natal family can be stimulated to escape ovulation suppression imposed by the mother by introduction of an unrelated male into the family. Female marmosets living with the natal family typically undergo reproductive suppression, failing to ovulate in the presence of the mother. The occurrence of such suppression is variable, however, as females have occasionally been reported to ovulate while housed with the natal group, and as both captive and wild groups have sometimes been reported to contain two breeding females. In this study, we are testing the hypothesis that females will escape from reproductive suppression in response to the introduction into the family of an appropriate mate, i.e., an unrelated adult male. Behavioral data are collected from anovulatory, post-pubertal females and their mothers, and blood samples are collected from daughters to monitor estradiol and progesterone levels, for 4 weeks while the family remains intact. The father is them removed and replaced by an unfamiliar adult male, after which behavioral and endocrine data collection continues for another 5 weeks. In other families, serving as controls, identical procedures are conducted except that the father is removed and immediately returned to his family. Preliminary results indicate that post-pubertal, anovulatory females can be stimulated to ovulate in 1-3 weeks following introduction of an unrelated male. Moreover, these females typically engage in sexual interactions with the novel male and eventually conceive. Escape from reproductive suppression can be accompanied by changes in intra-family social dynamics, including reversal of the dominance relationship between mother and daughter and rapid establishment of a dominance relationship between twin sisters. These findings indicate that reproductive failure in subordinate female marmosets may be mediated not only by rank-related suppression but also by the lack of appropriate, unrelated mates in the natal group. Moreover, they suggest that differences between marmoset groups in the number of breeding females may be attributable to differences in the relatedness of group members. Key Words reproduction, reproductive suppression, social relationships, novel male effect, sexual behavior, inbreeding, dominance